1. Field of the Invention
The present invention relates to a step-and-repeat exposure method and, more particularly, to an exposure method suitable for replacing reticles serving asmmasks during exposure of a plurality of shots on a photosensitive substrate.
2. Related Background Art
A conventional working mask, used with a proximity exposure apparatus or a projection exposure apparatus which has a magnification of one, has a plurality of circuit patterns arranged in a matrix form, and each circuit pattern corresponds to a single device chip. A reticle having an original image pattern enlarged 5 or 10 times the circuit patterns of one or a few chips is used to prepare the working mask. The reticle is mounted on a step-and-repeat exposure apparatus or a so-called photorepeater having a projection lens with a reduction ratio of 1/10 or 1/5. A mask blank (glass substrate) applied with a photosensitive agent is placed on an x-y stage of the photorepeater. The x-y stage is stepped in x and y directions to sequentially expose reduced images of 1/10 or 1/5 for the original image pattern on different areas on the mask blank.
In the photorepeater, unlike a wafer stepper, since an overlapping exposure is not performed basically, accuracy of a chip arrangement of the resultant working mask depends largely on stepping accuracy of the x-y stage. This also applies to the case wherein a first layer exposure (first print) is performed in the wafer stepper. This means that a working mask with high accuracy of a chip arrangement can be formed as long as stepping accuracy of the x-y stage is good.
However, chips including marks for mask alignment and chips including patterns for a test circuit different from those of real elements are often formed in the working mask in addition to chips including circuit patterns. This means that replacement of a plurality of reticles is required to expose a single mask blank.
This replacement of reticles often degrades accuracy of a chip arrangement in a single working mask. This is because an absolute alignment error of a reticle with respect to an exposure apparatus main body occurs after replacement. More specifically, an absolute positional difference between reticles before and after replacement directly influences accuracy of the chip arrangement. This will be described with reference to FIG. 6 which shows a shot arrangement on a mask blank MB. FIG. 6 illustrates a case wherein five reticles having different original image patterns A, B, C, D and E are replaced and exposed.
First, the reticle having the original pattern D is aligned on the apparatus using a predetermined reticle alignment optical system. According to design shot arrangement data (shot address), an x-y stage is positioned at each exposure position with reference to an origin O of an orthogonal coordinate system x-y. Positions are measured by a laser interferometric measuring device with accuracy of 0.02 .mu.m. Then, shots D.sub.1, D.sub.2, D.sub.3, D.sub.4 and D.sub.5 are sequentially exposed.
Next, the reticle is replaced with the one having the original image pattern E, which is aligned on the apparatus. Similarly, shots E.sub.1, E.sub.2 and E.sub.3 are sequentially exposed according to the shot address. In this case, since the origin serving as the reference of movement of the x-y stage remains the same as that of the previous reticle, shots E.sub.1, E.sub.2 and E.sub.3 are performed as illustrated by solid lines so long as the reticle after replacement has no alignment error. However, when an alignment error occurs, shots E.sub.1, E.sub.2 and E.sub.3 are deviated from the designed shot arrangement as illustrated by broken lines.
Thereafter, when the reticles having the original image patterns A, B and C, respectively, are exposed, shots A.sub.1, A.sub.2, B.sub.1, B.sub.2, C.sub.1, C.sub.2 and C.sub.3 are respectively deviated from designed shots arrangement as illustrated by broken lines because an alignment error of each reticle occurs. As described above, accuracy of a chip arrangement of the resultant working mask is degraded by an alignment error which occurs when a reticle is replaced.